1. Field of the Invention
The present invention relates to a method and apparatus for neutralizing pyrophoric gases. The invention can be used to neutralize, for example, pyrophoric gases used in the manufacture of semiconductor devices.
2. Prior Art
Many industries use pyrophoric gases for a variety of processes and operations. In the semiconductor industry, a variety of pyrophoric gases are used during the manufacture of semiconductor devices. These gases are termed pyrophoric due to their ability to ignite spontaneously upon contact with an oxidant such as oxygen. Thus, these gases may ignite upon contact with air, and if a pocket of pyrophoric gas contacts air, an explosion may result. The possibility of explosion is increased by the tendency of some pyrophoric gases to "self protect", wherein bubbles or pockets of the pyrophoric gas develop which prevents reaction or neutralization of the pyrophoric gas in a controlled manner.
Pyrophoric gases are usually used for the deposition of various layers or for introducing dopants into the various layers of a semiconductor device. For example, silane (SiH.sub.4) may be used along with oxygen to form a silicon dioxide (SiO.sub.2) layer in a chemical vapor deposition (CVD) system. Diborane (B.sub.2 H.sub.4), phosphine (PH.sub.3), and arsine (AsH.sub.3) may be used to add dopants to a layer. Silane also is used to form polycrystalline silicon layers as well as epitaxial, single crystal silicon in a variety of processes. Other commonly used pyrophoric gases include halogenated compounds such as dichlorosilane (SiH.sub.2 Cl.sub.2), among others. Generally, the processes which use these pyrophorics, such as CVD deposition, utilize a flow of the reactant gases through the reactor chamber. Normally, not all of the pyrophorics are spent in the process, and any remaining pyrophorics must be exhausted or pumped out of the reactor. This spent process gas must then be neutralized in a controlled manner so that it is not released into the atmosphere where it may cause a fire or an explosion upon contact with air. In addition to spent process gas, semiconductor manufacturing operations may have other sources of pyrophoric gas which must be neutralized. For example, the pyrophoric gas pump or pipe panel may be purged with nitrogen, and the exhaust line for the pump or panel must then be neutralized.
In the prior art, several systems exist for neutralizing a gas stream containing one or more pyrophoric gases. Many of these systems are "active" in that they provide a flame and reaction chamber through which the pyrophoric, as well as flammable and/or toxic gases in the spent gas, pass. The flame ensures combustion of nearly all pyrophoric gas, thus virtually eliminating the possibility of a sufficient accumulation of the pyrophoric gas to cause an explosion. To ensure a constant flame, a fuel source, such as hydrogen (H.sub.2) or natural gas (CH.sub.4) is ignited by, for example, spark plugs. Example of such systems include "The Sergeant", manufactured by Custom Engineered Material, Inc. and "The Guardian", manufactured by Hoechst Celanese Corp. These systems are often referred to in the art as "burn boxes."
Another prior art system is a dilution system. In a dilution system, the gas stream containing the pyrophoric gas is first diluted in an inert gas such as nitrogen (N.sub.2) in a ratio of approximately 100:1 (inert:spent process gas) by volume. This diluted gas stream is then combined with air in a reaction tube to allow the pyrophoric gas in the diluted gas stream to react with the oxygen in air. Because of the massive dilution of the spent gas stream prior to exposure to air, there is little possibility of explosion.
The aforementioned prior art systems have several drawbacks. The active systems are complex and expensive, requiring a large capital outlay. The high cost is especially significant since most facilities require several of these systems. In addition, the use of fuel such as H.sub.2 or natural gas adds additional ongoing operational expenses. Also, these systems require that the fuel source, which is a flammable gas, be piped to each system installed, thereby incurring additional capital outlays and presenting an additional fire hazard. The dilution type systems, while not as complex as the active system, are expensive to operate due to the large volume of inert gas used. Also, the dilution systems will not ensure complete reaction due to possible streamlining of the pyrophoric gas in the event of a sudden release, resulting in a potential for explosion.
What is needed is a system which can efficiently neutralize pyrophoric gas. The system should ensure complete or nearly complete neutralization of any pyrophoric gas contained in the gas stream to be neutralized. It is desirable that such a system be simple and inexpensive to build and operate. It is further desirable that the system does not require a fuel source to operate. Finally, such a system should be capable of handling a spent process stream that varies in pyrophoric gas concentrations from trace amounts up to several ten of liters per minute.